1. Field of the Invention
The present invention relates to an apparatus, which monitors the quality of an optical signal.
2. Description of the Related Art
As optical modulation, phase shift keying (PSK) or differential PSK (DPSK) has attracted attention, because PSK and DPSK modulation has an advantage that it is less subject to the dispersion of the optical fiber. Further demodulation is easy in case of DPSK. Multilevel modulation is also available, and quadrature PSK (QPSK) and differential QPSK (DQPSK) have been proposed in international patent publication WO02/51041 A2.
Meanwhile, Akihito Otani et al., “Demonstration of Far-end 160-Gb/s Waveform Measurement after 508-km Transmission in Field Trial without Traditional Clock Recovery”, ECOC 2006 Proceedings, Vol. 3, pp. 349-350 discloses a configuration which recovers a clock signal regardless of a symbol rate.